The fact is... They're just dogs. The reason I think they often aren't perfect for first time owners if that many people don't acknowledge or want to deal with propensity for dog aggression and prey drive. Because they're large terriers, many can't live safely with other dogs or small animals. They can range from completely dog friendly to totally dog aggressive, just like any other type of dog. They just happen to be more likely to err toward dog intolerance and aggression than others. They are not more likely than any other dog to be human aggressive. Sure, you could have temperament issues with any shelter mutt due to poor genetics. But I wouldn't be more worried about a BYB bully breed over a BYB of any other similarly sized dog or larger.